The high demand of polypropylene is increasing. This propylene growth combined with the scarcity of butenes (for making propylene by metathesis) has created a market for the utilization of pentenes to make propylene by metathesis. C5 olefins are available from two main sources. They are (i) C5 olefins available from stream cracker, and (ii) C5 olefins available from refinery, both containing significant amounts of isopentene along with the linear pentenes.
When linear pentenes are fed to a conventional metathesis reactor, the following reactions may occur:1-pentene-→2- pentene (Isomerization);  (a)2-pentene+ethylene-→1-butene+propylene (Metathesis);  (b)1-butene-→2- butene (Isomerization);  (c)2-butene+ethylene-→2 propylene (Metathesis).  (d)1-Pentene is isomerized to 2-pentene. The metathesis reaction of 1-pentene with ethylene is non-productive (products are same as reactants). The overall linear C5 olefin reaction can thus be shown as:1 linear pentene+2 ethylene-→3 propylene.
When isopentenes are fed to a conventional metathesis reactor, the following reactions may occur:2-methyl-2-butene+ethylene-→isobutene+propylene (Metathesis)2-methyl-1-butene-→2-methyl-2- butene (Isomerization)3-methyl-1-butene-→2-methyl-2- butene (Isomerization)The reactions of 2-methyl-1-butene and 3-methyl-1-butene with ethylene are non-productive. Thus, the metathesis of isopentenes results in only one mole of propylene, one-third the productivity of the linear pentene metathesis.
Typically, isopentenes content in these C5 streams is in the range of 40-60 wt % and is the most abundant species. As shown in the reactions above, the processing of isopentenes by metathesis results in the inefficient conversion of pentenes to propylene. Only one mole of propylene is formed from every mole of isopentene, as opposed to three moles of propylene from every mole of linear pentene.
Table 1 shows the boiling points of the linear pentenes and the isopentenes.
TABLE 1Boiling points of PentenesBoiling PointPentene(° C.)Type3-Methyl 1-Butene26.00Isopentene1-Pentene29.95Linear Pentene2-Methyl 1-Butene31.20Isopentenet-2-Pentene36.30Linear Pentenec-2-Pentene36.90Linear Pentene2-Methyl 2-Butene38.55Isopentene
As can be seen from the staggered boiling points of the linear pentenes and the isopentenes, a simple separation of the linear pentenes from isopentenes by distillation is not easily achievable.